Do you regard relativistic effects to be physically real or just apparent (kind of like optical illusion caused by distorted information one is receiving)? To prevent endless entanglement in "define physically real" arguments imagine this: two spaceships moving inertially relative to each other in space. Both see the other one as shortened in direction of movement. But there is an alien from parallel universe who has the ability to shoot his laser measurer with infinite speed in our universe without affecting it in any way. What would he say about the length of both ships? Would he find them to be the same?